This invention relates to illumination optical systems, exposure apparatus and exposure methods. In particular, this invention relates to exposure apparatus for manufacturing micro devices, such as semiconductor elements, imaging elements, flat panel displays such as liquid crystal display elements and thin-film magnetic heads, using lithographic processes.
In typical exposure apparatus of this type, a secondary light source, which functions as a substantially planar light source composed of a plurality of light sources, is formed by a light beam from a light source that is transmitted through a fly's-eye lens that functions as an optical integrator. The light beams from the secondary light source are incident to a condenser lens after being regulated through an aperture diaphragm positioned near a rear focal plane of the fly's-eye lens.
The light beams converged by the condenser lens illuminate superimposingly a mask on which a predetermined pattern is formed. The light transmitted through mask pattern forms an image on a wafer through a projection optical system. As a result, the mask pattern is projected and exposed (transferred) onto the wafer. Because the pattern formed on the mask is highly integrated, it is necessary to obtain a uniform illumination distribution on the wafer in order to accurately transfer the micro patterns onto the wafer.
Accordingly, attention has been given to a technology in which a circular secondary light source is formed on the rear focal plane of the fly's-eye lens, and illumination coherency σ(σ=an aperture diaphragm diameter/a pupil diameter of a projection optical system; or σ=an exit-side numerical aperture of an illumination optical system/an incident-side numerical aperture of the projection optical system) is changed by varying a size of the secondary light source. Moreover, attention has been given to a technology in which a depth of focus and resolution of the projection optical system are improved by forming an annular or quadrupole secondary light source.
In the above-described conventional exposure apparatus, normal circular illumination is performed with a circular secondary light source, and modified illumination (annular or quadrupole illumination) is performed with an annular or quadrupole secondary light source, depending on the characteristics of the mask pattern. However, it is generally the case that, regardless of the characteristics of the mask pattern, the mask is illuminated with light whose polarized state is not changed, that is, light in a nonpolarized state. Appropriate illumination conditions that are required to precisely transfer the mask pattern onto the wafer are not always secured.